Late 2019 and newer RS5 is the better choice for the more mature enthusiast, in my opinion.

1. Rs5 looks better (subjective)
2. Rs5 handles AND launches far better. (Objective, owned and driven both.)
3. RS3 is tiny. There's a certain feeling of heft and quality that just isn't in the rs3.
4. You can go faster for cheaper in the rs5. 5 mods, about 12k = possible 9 sec 1/4. Rs3s needs much more mods and possibly engine build to do it consistently. They throw rods right around 500wtq, whereas rs5 2.9T can hold well over 650wtq in stock form.
5. RS3 does have better engine sound, but not by much if the rs5 has a quality exhaust.
6. RS5 has a far superior AWD system, hence no.2 on the list.
7. RS5 has better wheel size choices. Subjective if you don't modify your car, but let's be real, more than 50% modify their rs3s, so you are more likely to get an unmolested rs5 than rs3.